Designing a golf driver requires the balancing of many factors within the framework dic- tated by the rules of the US Golf Association (in the USA and Mexico) and the R & A (worldwide). While the rules of the game provide hard constraints on several aspects of the design of the club, there remain many trade-offs to be made in the design process. Each year, club manufacturers release new clubs and make new claims about the scientific im- provements they have made that will allow golfers to hit the ball further and straighter. In this section, several different design decisions and manufacturers claims will be described along with the possible and expected outcomes of those decisions. These decisions and claims will be revisited and analyzed within the context of the model presented in Chapter 3 in Chapter 4.
2.2.1
Club Mass
One of the more recent claims of golf club manufacturers is that a lighter club would result in longer drives [31]. The claim made was that by decreasing the club mass 10 g, the golfer would be able to swing the club 1 mph (0.45 m/s) faster, resulting in longer drives. The trade off here is that by decreasing the club mass, the momentum of the club is not necessarily increased overall by an increase in clubhead speed and the momentum of the clubhead before impact that controls the amount of energy that can be transferred to the ball. It is likely that there is a sweet spot for club mass for each individual golfer and finding that spot is one of the challenges in designing a club.
2.2.2
Clubhead Moment of Inertia
Due to the gear effect of the ball interacting with the club (see Section 2.1.4, the amount of angular velocity imparted to the clubhead during impact is important in determining the side-spin of the ball. By increasing the moment of inertia (MOI) of the clubhead, the angular velocity of the clubhead is reduced and the amount of spin imparted to the ball is similarly reduced. By increasing the MOI of the clubhead about its vertical axis, manufacturers have created drivers that can hit the ball straighter for off-centre impacts on the club. This led to the creating of a rule by the USGA that no club may have a MOI greater than 5900 g cm2 [25].
Even without this rule, improvements to the club made by increasing the clubhead MOI were shrinking as the increase in MOI makes it more difficult for the golfer to close the clubface at impact. Since the MOI is larger, more torque is required from the forearms and hands to rotate the club to the appropriate angle. There is clearly a tradeoff between increasing the MOI to reduce the spin and decreasing the MOI to increase the controllability of the club.
2.2.3
Clubhead Centre of Mass Position
The ideal centre of mass position of the clubhead is also a matter of debate. While most people agree that then the centre of mass of the clubhead should be low, to decrease the amount of backspin on the ball, it is unclear whether it should be low and close to the clubface or low and far from the clubface. In particular the TaylorMade SLDR driver released in 2013 claimed that a low and forward centre of mass location would provide better launch conditions.
The centre of mass should be low so that the gear effect of the ball striking above the centre of mass reduces the backspin of the ball as most golfers tend to hit with too much backspin for an optimal flight (see Figure 2.11b). Striking above the centre of mass causes the clubface to rotate upwards and reduces the backspin on the ball. Moving the centre of mass forward or backwards normal to the clubface should not change this effect as the moment arm of the impact force will not change. It’s unclear whether this movement might have other effects and this should be further investigated. Moving the centre of mass forward horizontally relative to the ground should reduce the backspin as the moment arm of the impact will be increased.
2.2.4
Shaft Flexibility
The shaft of the golf driver bends forward at impact so that the clubhead strikes the ball while angled slightly upwards. This can help to reduce the backspin of the ball and increase the launch angle to improve the carry. The common wisdom is that a golfer with a faster swing speed needs to use a stiffer club and a golfer with a slower swing speed should use a more flexible club. It’s important for a golfer to select the correct shaft flexibility for their particular swing and this selection is one of the parts of a traditional shaft fitting performed by a golf pro.
While shaft flexibility does not involve design trade offs in the traditional sense the selection of the appropriate shaft can be difficult and deserves consideration. Any single shaft is unlikely to be correct for all golfers, but the question of fitting a shaft for a particular golfer is an interesting one. In a world where more goods will be personally manufactured for individual consumers the possibility of designing the shaft flexibility directly for one particular golfer using a computer model is attractive. While the model
presented in this paper is not subject-specific it can be used for testing different flexible shafts and optimizing the stiffness.